When a voltage is applied to an organic electroluminescence device (hereinafter referred to as an organic EL device), holes are injected from an anode into an emitting layer and electrons are injected from a cathode into the emitting layer. The injected holes and electrons are recombined in the emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission principle, in a fluorescent EL device which uses emission caused by singlet excitons, an internal quantum efficiency of the organic EL device is believed to be limited to 25%. On the other hand, it has been known that the internal quantum efficiency can be improved up to 100% under efficient intersystem crossing from the singlet excitons in a phosphorescent EL device which uses emission caused by triplet excitons.
A technology for extending a lifetime of a fluorescent organic EL device has recently been improved and applied to a full-color display of a mobile phone, TV and the like. However, an efficiency of the fluorescent organic EL device is required to be improved.
Based on such a background, a highly efficient fluorescent organic EL device using delayed fluorescence has been proposed and developed. For instance, Patent Literatures 1 and 2 each disclose an organic EL device using TTF (Triplet-Triplet Fusion) mechanism that is one of mechanisms for delayed fluorescence. The TTF mechanism utilizes a phenomenon in which a singlet exciton is generated by collision between two triplet excitons.
By using delayed fluorescence caused by the TTF mechanism, it is considered that an internal quantum efficiency can be theoretically raised up to 40% even in fluorescent emission. However, as compared with phosphorescent emission, the fluorescent emission still needs to be improved in efficiency. Accordingly, in order to further improve the internal quantum efficiency, an organic EL device using another delayed fluorescence mechanism has been studied.
For instance, TADF (Thermally Activated Delayed Fluorescence) mechanism is used. The TADF mechanism utilizes a phenomenon in which reverse intersystem crossing from triplet excitons to singlet excitons is generated by using a material having a small energy difference (ΔST) between the singlet level and the triplet level. An organic EL device using the TADF mechanism is disclosed in, for instance, non-Patent Literature 1. In the organic EL device of non-Patent Literature 1, a compound having a small ΔST is used as a dopant material to cause reverse intersystem crossing from the triplet level to the singlet level by heat energy. It is considered that the internal quantum efficiency can be theoretically raised up to 100% even in fluorescent emission by using delayed fluorescence by the TADF mechanism,
Non-Patent Literature 2 discloses an organic EL device having a doped film including a specific host material and a specific compound having a Spiro skeleton as a dopant material. The organic EL device exhibits a high external quantum efficiency by using the TADF mechanism.